It is known to enamel parts made of heat-resistant metals, in particular iron and iron alloys, which are exposed to high-temperature gases of combustion, such as combustion chambers, heat exchangers and exhaust systems, including mufflers.
However, the enamels used up to now for this purpose have had several drawbacks. Their heat resistance has been inadequate. Due to the reboiling effect, small bubbles would form in the enamel in the heated areas. Temperature changes have resulted in mechanical damage. There have been problems of adherence at welded seams. Sharp edges on the parts, as around punched holes, have remained insufficiently coated with enamel.
In special enamels of conventional composition based on silicates, as for selfcleaning ovens or for gas-heater flues and heat exchangers, for example, use has been made of aluminum powder. However, enamel slip of such composition containing aluminum powder continually evolves hydrogen, which makes the enamel frothy and causes trouble when it is ground. This has in the past occasionally resulted in dangerous oxyhydrogen gas explosions in the preparation of the slip and in its application. Metallic silicon behaves in a similar manner.